Vapor stripping method

ABSTRACT

Coatings, such as paint or oil, are stripped from the surface of a coated object by circulating a stream of a stripping composition in a gaseous state at ambient temperature and pressure into contact with the surface. The stripping composition is normally a liquid at ambient temperature and pressure and has a partial pressure of at least 5 mm of Hg. at ambient conditions. The contacting procedure is conducted substantially in the absence on the surface of liquid condensate of the stripping composition.

BACKGROUND OF THE INVENTION

The present invention relates to a method for stripping protectivecoatings from coated objects. More particularly, the present inventionconcerns a method for stripping a protective coating obtained fromcompositions based on organic resins and/or prepared with organicvehicles, such as paint, shellac, varnish, lacquer and the like. Themethod of the invention is especially useful for removing such coatingsfrom objects have irregular surfaces and from large surfaces, includingvertical and inclined surfaces in the interior of large constructions,such as storage bins and tanks on land and the holds and ballast tanksof ships.

Commonly, paint is stripped from painted objects by application of anorganic or inorganic solvent or mixture thereof. As discussed inKirk-Othmer's ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Vol. 14, pps.485-493, 2nd Edition, John Wiley and Sons, 1967, organic paint removersgenerally fall into three classes: compositions based on chlorinatedhydrocarbon solvents, compositions consisting of mixtures of othersolvents and removers based on aqueous solutions or dispersions ofphenols and/or organic acids and other compounds. Inorganic strippers,such as an aqueous solution of caustic soda and in some cases, mineralacids are also used, particularly for industrial applications.

Among the chlorinated hydrocarbon solvents, methylene chloride(dichloromethane) has been found to be particularly effective andformulations of methylene chloride suitable for application by dip,brush, spray and delivery from aerosol cans are known. Such compositionsalso usually contain additives including thickeners, evaporationretarders and detergents.

Organic solvent formulations for stripping paint and other coatings maybe of the "scrape off" type or "flush off" type. Generally, thestripping composition is applied to the coated object by one of theforegoing methods and allowed to stand for some time, after which, thecoating which has become swollen and/or softened is removed from thesurface, by scraping, in the case of "scrape off" formulations or byflushing with water and/or by wiping with a damp rag in the case of"flush off" formulations.

The foregoing methods are relatively expensive, since the organicsolvent, except in the case of application by immersion, is not in aform which can be recovered practically. Moreover, all of the foregoingprocedures are generally impractical and prohibitively expensive wherelarge surfaces are involved. In addition, extreme safety measures wouldbe required to effectively treat large surfaces by any of the foregoingmethods; the measures necessary to protect personnel from strippingchemicals, most of which are exceedingly toxic, essentially prohibittheir use for stripping large objects. Another important problem withthe foregoing procedures is the difficulty of completely removing theadditives from the stripped surface, particularly the waxes used asevaporation retarders in formulations of organic chemical strippingcompositions; any residual wax interferes with the adhesion ofsubsequent coating of the surfaces.

Processes have also been described in U.S. Pat. Nos. 2,689,198 to Judd;3,794,524 to Nogueira, et al. and 3,832,235 to Cooper, et al. whereinpaint is stripped from a relatively small object by contact with thevapors from a boiling solvent composition. In these processes the hotvapors condense to liquids on the painted surface. The resultant hotliquid not only provides a high local concentration of the paintstripping composition, but it also washes off any soluble components ofthe coating or breakdown products thereof.

Such methods are not applicable for removing protective coatings fromextensive surfaces because the cost of heating a sufficient amount ofsolvent to reflux is prohibitive and moreover, expensive equipment wouldbe needed to carry out such an operation on a large scale. Furthermore,in some constructions, such as large metal tanks and ships, even amoderate temperature differential from one part of the construction toanother can be harmful.

It is the current industrial practice to remove paint and otherprotective coatings from large tanks and other large constructions bythe slow, unpleasant and relatively expensive procedure of abrasiveblasting. It is important that the ballast tank of a ship, which usuallycarries ballast water, be rust-proof. To this end, the ballast tank iscoated with a layer of paint. If the paint coating blisters or fails inany way, it is necessary to remove the paint from the interior of theballast tank and repaint, to avoid the possibility of rust and eventualholes. This is especially important for ships which carry liquifiednatural gas. The ballast tank of a ship may have a capacity as large asone million gallons or more and often has a complex "honeycomb"configuration which makes it difficult and laborious for a blaster towork through. Also, the removal of the large amount of blasting gritneeded is costly.

To date, even though abrasive-blasting has severe disadvantages, it isthe only procedure in use for removing paint from large surfaces.

SUMMARY OF THE INVENTION

The principal object of the present invention is the provision of amethod of stripping a protective coating from a coated object by aneconomical procedure which avoids the problems associated with knownstripping procedures.

Another important object of the present invention is the provision of amethod for stripping a protective coating from extensive surfaces by aneconomical and safe procedure.

A further important object of the present invention is the provision ofa method for stripping protective coatings which avoids the use ofadditives which may interfere with subsequent recoating of the surface.

A further object of the present invention is the provision of a methodfor stripping protective coatings from extensive surfaces in theinterior of a large construction, such as storage tanks, ballast tanksand holds of ships.

Still another object of the present invention is the provision of aneconomical method for stripping protective coatings from surfacesirrespective of the shape, complexity or inclination thereof.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

With the above and other objects of the invention in view, our inventioninvolves the novel method for stripping a protective coating from acoated surface by contacting the surface with a stripping composition inthe gas phase capable of destroying the adhesion between the coating andthe surface, substantially in the absence of liquid strippingcomposition condensate on the surface.

DESCRIPTION OF THE INVENTION

We have discovered that protective coatings may be substantiallyloosened and in many cases completely stripped from surfaces, solely bythe action of the vapors of a stripping composition. By protectivecoatings is meant any coating based on an organic resin or organicvehicle, such as paint, shellac, varnish, lacquer and the like, which isapplied to a surface such as metal or wood for the protection and/orenhancement thereof. The process can also be used to remove coatings notusually called protective, which are included within the meaning ofprotective coatings in this disclosure. These include residual crudeoil, Bunker C fuel oil, and the like which have to be completely removedfrom the surfaces of holds or tanks when they are to be filled with adifferent substance which would be contaminated by these residues.

In accordance with our invention, a surface which is to be stripped iscontacted with the vapors of a stripping composition until the adhesionbetween the coating and the surface is destroyed. The gaseous strippingcomposition is introduced into contact with the coated surface at aconcentration, pressure and temperature such that substantially nocondensation of the gaseous composition occurs on the coated surface andthus the process takes place substantially in the absence of liquidcondensate. Even complex and very large surfaces can be treated readilyby the procedure.

Depending on the particular coating, the particular strippingcomposition, the size of the stripping surface and the equipment used,the time required to destroy the adhesion of the paint ranges from about1 hour to a few days. Preferably the process is carried out at aboutambient temperature. There is no theoretical lower temperature limit forour process. As long as the stripping chemicals have a little vaporpressure, the process works, but the rate slows down at low vaporpressure. The present process is effective at a temperature as low asabout 32° F. (0° C.); however, the action of the gaseous strippingcomposition is faster at higher temperature.

It is a particular advantage of our process that it is unnecessary toheat the gaseous stripping composition to reflux and in most cases it isunnecessary to heat the gaseous stripping composition at all, since mostpreferably the process is carried out essentially at the temperature ofthe environment.

When carrying out the process at about ambient temperature, it ispreferred that the ambient temperature be at least about 32° F. and evenmore preferably at about 45° F., otherwise the process may beinconveniently slow for some paint; although even at an ambienttemperature below 32° F., the present process may be preferable to otheravailable process such as abrasive blasting.

When the surface which has been thus contacted is freed from the gaseousstripping composition, by air drying or other convenient means, it isfound that in many cases, the coating, particularly a paint coating, haseither fallen off completely or can be brushed off readily leaving onlysmall specks of paint. In most cases, the surface which has beencontacted with the gaseous stripping composition is about 75-95% free ofvisible coating residue and sometimes no residue is visible. Usually,the surface can be recoated without further treatment. However, evenwhen an objectionable amount of coating remains, the surface can beabrasive-blasted so as to be 100% clean, i.e. a "white metal blast", ina substantially shorter time than that required to obtain a surfacewhich is 100% clean by abrasive-blasting alone. Often, after the surfacehas been contacted with the gaseous stripping composition in accordancewith our invention, sand blasting to achieve a 100% clean surface can beachieved in about 5% to about 20% of the time normally required. Inalmost all cases, the coated surface which has been vapor treated inaccordance with our invention can be abrasive-blasted 100% clean in nomore than about 50% of the time normally required.

During the process of our invention, it is believed that the vapor isabsorbed into the coating causing the coating to undergo physicalchanges and to break loose from the substrate. Many epoxy, alkyd,polyurethane and polyester coatings form dry flakes which can be readilyand economically disposed of. This is a particularly unexpected furtheradvantage of treatment with a gas phase stripping compositionsubstantially in the absence of liquid condensate in accordance with ourinvention. When a surface is treated with liquid stripping composition,which occurs when the vapors of a refluxing composition condense toliquids on a cooler painted surface; the liquid stripping compositionleaches out components of the coating, making a sticky mess, thecleaning of which is difficult and substantially less economical thanthe removal of dry flakes.

It has been found that some coatings of chlorinated rubber in particularmay be turned into a powder, rather than flakes by a treatment inaccordance with the invention; however, the powder is also readilydisposed of. In one case, a chlorinated rubber paint was liquified,solely by the vapors of an organic stripping fluid. However, even insuch a situation, treatment in accordance with our invention ispreferable to abrasive-blasting, which is especially slow with flexiblecoatings such as rubbers. In the treatment of other chlorinated rubbercoatings, it has been found that the coatings are embrittled but are notsignificantly removed from the surface; however, abrasive-blasting,hydroblasting or brushing removes the treated coating substantiallyfaster after treatment with a gaseous stripping composition inaccordance with our invention, than in the absence thereof.

The method of the present invention is particularly advantageous forremoving coatings from surfaces within a sealed or sealable container.Moreover, the present method is as effective with regular surfaces aswith regular surfaces and the surface may have any inclination;relatively large surfaces, such as the inner surfaces of storage tanksmay be treated readily in accordance with the present process.

In another preferred embodiment of our invention, the surface to betreated is substantially sealed from the atmosphere to form a paintstripping zone. A stream of paint stripping composition in the gas phasepreferably close to ambient temperature is introduced into the paintstripping zone into contact with the painted surface. In the case of astripping composition which is liquid at ambient temperature, the gasstream can be generated conveniently by blowing air over the surface ofthe liquid stripping composition in an evaporator, which is connected tothe paint stripping zone by means of chemical resistant conduits. If astripping compound is used which is normally a gas at ambienttemperature, the gas may be introduced directly into the paint strippingzone without use of an air blower. The paint stripping zone ispreferably provided with a return conduit to the vacuum side of the airblower, which allows the air and the gaseous stripping composition to berecirculated. As the partial pressure of the gaseous strippingcomposition increases in the paint stripping zone, air is bled from thestripping zone; normally, the density of the gaseous strippingcomposition is greater than that of air, so that the air can usually bebled out near the top of the stripping zone. This allows higherconcentrations of the gaseous stripping composition to be reached andcan be used to prevent an undesirable rise in pressure from occurring.

The evaporators may be heated to replace the heat of vaporization of theliquid stripping composition and to prevent the liquid strippingcomposition from cooling excessively. But, it is preferable that thegaseous stripping composition be at or near ambient temperature in theconduit and stripping zone.

It is also possible to evaporate the liquid stripping composition insidethe paint stripping zone in which case, a special evaporation zone maybe eliminated.

Means for circulation of the gaseous stripping composition are desirablyincluded in the paint stripping zone. The efficiency of the presentprocess is increased and the time required to destroy the adhesion ofthe coating and the surface is decreased when the gaseous strippingcomposition is thoroughly circulated throughout the stripping zone. Forthis purpose, efficient gas pumps and/or blowers may be employed.

In the stripping zone the gaseous stripping composition is absorbed intothe coating to be stripped, whereby the coating undergoes physicalchanges and breaks loose from the substrate. The gaseous strippingcomposition is then pumped from the stripping zone and desorbed from thecoating as the partial pressure of the stripping composition drops. Thegaseous stripping composition may then be recovered by condensation orvented. The cost of the chemical components of the stripping compositionis minute compared to the cost of abrasive-blasting. However, it is notdifficult to recover most of the stripping composition used in thepresent process and recovery avoids air pollution. Air is bled into thepaint stripping zone through a vacuum release valve during the removalof the gaseous stripping composition to avoid creating a possiblydangerous vacuum. In most cases and particularly in the case of mostepoxy coatings, after the removal of the gaseous stripping composition,the coating is in the form of dry flakes, mainly on the floor of thepaint stripping zone; the dry flakes can be quickly and economicallyremoved, for example by a vacuum cleaner.

The gaseous stripping composition may be continuously introduced intothe stripping zone and it is also preferable, especially when time is afactor, to continuously remove air from the top of the stripping zone,which may be accomplished through a pressure relief valve set at about1-2 psi until the air has been substantially removed and the highlyconcentrated vapors of the more dense stripping compounds are vented.The vented chemical vapors can be condensed for reuse. The gaseousstripping composition may also be continuously withdrawn from thestripping zone, condensed, or retained in the gaseous state and recycledback to the paint stripping zone or where two or more areas are beingstripped, the gaseous stripping composition withdrawn from one strippingzone may be circulated to another stripping zone. In large scaleoperations, blowers are used, in order to distribute the vaporsthroughout the structure in a reasonable time.

In the event that it is impractical to visually observe the condition ofthe coating in all parts of a complex construction once it has beensealed, properly located viewports and/or fiber optic devices usuallycan be conveniently employed to the extent necessary to determine whenthe process is complete.

No single gaseous compound or mixture thereof has yet been found whichis ideally suited to the many types of protective coatings in use today.Normally, a few simple experiments will enable one of ordinary skill inthe art to determine an effective compound or mixture. Organic andinorganic compounds known to be useful for stripping paint, shellac,varnish and the like, which have a partial pressure of at least about 5mm. Hg at ambient temperature can be used in our process. In practice,we prefer to use mixtures containing a relatively high percentage oflower chloroalkanes, particularly chloroalkanes containing 1-3 carbonatoms and 1-3 chlorine atoms. Methylene chloride is an especially usefulstripping agent from the point of view of effectiveness, as well as ofsafety and economy. However, other chloroalkanes, such as1,2-dichloroalkanes and chloroform are also advantageous. Not only aresuch chloroalkane mixtures usually more effective and economical, butalso fire and explosion hazards may be reduced or eliminated andtoxicity greatly reduced. Stripping compositions containing methylenechloride in an amount of about 25 to 100% by volume, more preferablycompositions containing methylene chloride as the principal ingredientand even more preferably, particularly for economy and safety,compositions containing about 70, 80 or 85 to 95% of methylene chlorideare used.

Compounds which we have found to increase the effectiveness of methylenechloride and other lower chloroalkanes with various coatings includewater, lower carboxylic acids, such as formic acid, ammonia,lower-alkylamines, lower alkanols, and lower alkyl ethers, esters,ketones, nitriles, amides, lower alkanes, arenes, such as benzene andlower-alkyl and halogen substituted benzene, and volatile inorganicacids. The term "lower", refers to a compound having one to four carbonatoms. In general, vapor phase compositions, which contain about 70 to95% of methylene chloride, at least about 1% water and about 4 to 29% byvolume of other compounds, such as those just listed are most effective.

For example, gaseous mixtures of methylene chloride and commercialformic acid (85-90% aqueous) in proportions of about 90-97% of methylenechloride to 3-10% of formic acid are very effective for destroying theadhesion of a variety of epoxy coatings to sand blasted steel as well aswood. Several other types of paint, including an alkyd, a polyurethaneand a bituminous aluminum paint have been 100% delaminated with agaseous mixture of methylene chloride/formic acid/water in proportion byvolume of about 95% of methylene chloride and about 5% of 85% formicacid. Such a mixture removed a ketimine epoxy coating substantiallycompletely in about 48 hours; on the other hand, a chlorinated rubbercoating was converted into a soft powder and a thick coating of glassflake polyester was only partially removed with this mixture of vapors.

It has also been found that lower alkyl amines are powerful activatorsfor methylene chloride in the gas phase; compositions containing about70 to 90% by volume of methylene chloride and 10 to 30% of 33 to 75%aqueous ethyl amine are particularly useful. A mixture of about 10% byvolume of monoethylamine (33% aqueous) and 90% by volume of methylenechloride is more effective than a gaseous mixture containing about 95%of methylene chloride and about 5% of formic acid (85% aqueous) forcertain polyurethane, alkyd and epoxy coatings. An epoxy coating whichwas almost unaffected by the formic acid-methylene chloride mixture hasbeen substantially completely delaminated by means of a gaseous mixturecontaining about 70% of methylene chloride and 30% of monoethylamine(33% aqueous). Diethylamine has also been found to activate methylenechloride, but generally appears to act more slowly than ethylamine.

Small molecules with dipole moments and acidic or basic character seemto be the most generally useful alone and in combination with methylenechloride for stripping paint. Thus compositions containing about 70 to95% of methylene chloride, about 1% of water and 4 to 29% of eithermethyl alcohol or methyl ethyl ketone are also useful.

In some cases, compounds in the gas phase appear to have a synergisticeffect with respect to methylene chloride. For example, the gaseousmixture of formic acid and methylene chloride and the gaseous mixture ofmethylene chloride, diethylamine, methanol and water are surprisinglyeffective with respect to an arylamine epoxy coating on metal. It issurprising that the latter four-component system has a substantiallyfaster action than that of three-component mixtures wherein eithermethanol or the amine is omitted. The rate of delamination of an epoxycoating using 85% formic acid vapors was greatly accelerated by theaddition of methylene chloride vapors, yet this epoxy was not strippedby methylene chloride vapor alone.

In the event that a gaseous stripping composition is chosen whichcontains two or more components which do not form a homogeneous solutionin the liquid phase, it is preferable to have separate evaporators foreach of such compounds.

The particular amount of stripping composition used varies widely,depending upon the nature and thickness of the coating, the ambienttemperature and the particular stripping composition selected, as wellas the volume of the stripping zone and the area of the coated surfaceto be treated. Broadly speaking, the ratio of the weight of strippingcomposition used to that of the coating removed may be from about 0.5:1to as much as about 4:1. We prefer to use enough stripping compositionto ensure that the gaseous stripping composition in the stripping zoneis close to saturation with respect to each component of the strippingcomposition; most preferably the gaseous stripping composition is about90-99% saturation, so that the coating absorbs the maximum amount of thevapors possible at the prevailing temperature or at the lowest ambienttemperature expected during the process. When most of the air is bledoff, as in the recycling variation of our process, the atmosphere in thestripping zone is mainly stripping gases. However, the combined partialpressure of the stripping gases is such that substantially no liquidcondenses during the treatment.

The method of our invention is particularly advantageous in reducing thecost required to strip unsatisfactory coatings from very large surfaces;the time and material required for the usual procedure of abrasivestripping can be eliminated or substantially reduced.

Provided that the area to be stripped can be substantially sealed fromthe atmosphere, there is no practical upper limit to the size orcomplexity of painted structures which can be treated with gaseousstripping compositions in accordance with our invention. The fact thatthe present procedure neither endangers nor damages the structure bypressure or temperature change is an important advantage of the presentprocess. Moreover, we have observed no corrosion problems whatsoeverwith respect to metal surfaces using the preferred strippingcompositions as disclosed above.

Our method is very economical, since the cost of the chemicals iscurrently low and moreover, most of the chemicals can be recovered bycondensation for reuse. The equipment needed is commercially availableat reasonable cost and the manpower requirements are low.

Another important advantage of our paint stripping procedure is thatpersonnel need not be exposed to the chemical stripping agents; thechemicals can be transferred from shipping containers to the strippingsystem with little or no exposure to the atmosphere.

The method of our invention is especially useful for removing paint frominterior surfaces of ballast tanks of ships and large tanks used forstoring or processing water, beverages and chemicals. Removal of paintfrom such large areas with liquid chemicals is clearly impractical;applying a stripping fluid by any of the usual methods is hazardous,time consuming, expensive and may leave undesirable residues.

The following examples further illustrate the present invention, butmust not be construed as limiting the invention in any mannerwhatsoever. In the following examples, as well as in the disclosure as awhole, all proportions of stripping components are by volume unlessotherwise indicated; relative proportions of solvents to paint coatingare by weight.

EXAMPLE 1

A 16 sq. cm. area of a steel plate which has been abrasive-blasted andspray painted with two coats (12 mils) of an epoxy manufactured byCarboline Co. was grit-blasted to a near white metal condition with asmall Speedaire "Sandblasting Gun" (3/16 i.d. nozzle) using "Stanblast"grit (furnace residue) and a pressure of 80 psi. The time required was85 seconds.

Another portion of the painted surface was placed over a plastic beakercontaining methylene chloride (9 ml.) and 90% formic acid (1 ml.); after14 hrs. exposure to the vapors at 73° F. most of the exposed epoxycoating had delaminated in fragments and fallen into the beaker. Theplate was allowed to stand in air (73° F.) for four hours. A 16 sq. cm.area of the treated surface was then grit-blasted to white metal usingthe afore-described equipment and conditions. This took only a fastsweep of not over 5 seconds, only 6% of the time needed for theuntreated coating.

EXAMPLE 2

A test panel coated with an arylamine epoxy made by Southern ImperialCoating Corporation required 180 seconds to blast a 16 sq. cm. area towhite metal using the same equipment as in Example 1. When exposed tothe vapors of 99% methylene chloride - 1% water in a thin layerchromatography (TLC) chamber for 47 hrs. at 73° F. the coating appearedlargely separated from the metal. After standing in air 8 hours a 16 sq.cm. area was grit-blasted for 25 seconds; about 90-95% of the surfacewas free of visible paint residues. Another 25 seconds blasting took itto white metal (no visible paint residue). A 72% reduction in blastingtime was realized.

The same plate was next exposed to the vapors of 95% MC - 5% formic acid(88%) in a TLC chamber for several hours at 72° F. and aired for 20minutes. The blasting time to white metal for 16 sq. cm. was reduced to7.9 sec., 4% of the time needed for the untreated epoxy. Similar resultswere obtained when the preliminary treatment with wet methylene chloridevapor was omitted. When a steel panel coated with Bunker C fuel oil wastreated similarly the oil ran off the panel completely in 30 minutes.

EXAMPLE 3

A sandblasted steel panel coated with an 8-9 mils of an aryl amine epoxy(Southern Imperial Coating Corporation #1204) was placed in a 148 ml.screw capped jar at 72° F. with 1.32 g. of methylene chloride, 0.03 g.of tap water, and 0.067 g. of 85% formic acid. The panel was supportedabove the liquids on wire gauze. Within an hour the coating was almostcompletely delaminated. After 2 days the coating fragments, which hadcompletely fallen off the panel were removed, and dried to constantweight at room temperature. It was found that 2.51 g. of dry paint hadbeen removed by the vapors from 1.44 g. of chemicals.

EXAMPLE 4

A panel coated with a polyurethane paint (Southern Imperial CoatingCorp. #4311) was exposed at 72° F. to the vapors of a mixture of methylethyl ketone, water, and methylene chloride in the ratio of 5:1:94 byvolume was 100% delaminated; the coating fell off in one piece in 2days.

EXAMPLE 5

A steel panel with a heavy coating of a glass flake polyester (18 mils)was exposed to the vapor mixture of methanol, water, and methylenechloride (5:1:94) for 48 hours at 72° F. Most of the polyester fell ofleaving a thin ragged soft coating adhering weakly to the metal. Asimilar experiment with a 25 mil coating treated with 99% methylenechloride - 1% water gave a similar result except that 7-12 ml. ofbrittle residual polyester remained. Blasting times were not measuredbut would obviously be reduced by at least 50%.

EXAMPLE 6

A panel coated on both sides with 6 mils of an alkyd paint on 4 mils ofa red primer was put on a wire screen in a desiccator containing 5 ml.of 33% aqueous ethylamine and 45 ml. of methylene chloride for 48 hours.The alkyd coating was about 90% delaminated in 8.5 hrs. and 100% removedin 48 hrs. A faint color due to a little primer left in the anchorpattern was observed.

EXAMPLE 7

A steel panel coated with a 2-coat arylamine epoxy paint (10 mils) wasexposed at 63° F. to the vapors from a mixture of 95% methylene chlorideand 5% formic acid (90% aqueous) for 16 hours in a covered cylinder.When the panel was lifted out of the cylinder, the coating slipped offleaving only a few fragments of the primer coat on the metal.

EXAMPLE 8

(A) Two steel panels;

(1) coated with an aromatic amine type epoxy (Carboline)

(2) coated with a polyamide type epoxy (Southern Imperial Coating Corp.#1201)

are placed inside a one-gallon receptacle which has been fitted withvapor supply and vapor withdrawal lines (1/4 inch i.d.). The receptacleis then substantially sealed from the atmosphere. A small squirrel cageblower (Dayton 4C 047) sealed in a container and connected to the vaporsupply and vapor withdrawal lines is used to evaporate a mixture of 93%methylene chloride, 2% water and 5% formic acid (85% aqueous) containedin a round bottom flask and to force the resultant gaseous mixturethrough the vapor supply line into the one-gallon receptacle, out of thecontainer through the vapor withdrawal pipes and back to the blower, sothat the mixture of vapors is continuously recirculated. After six hoursof contact with the circulating vapors at 69° F., panel (1) coated witharomatic type epoxy is completely delaminated; however, panel (2)painted with the polyamide type epoxy is not visibly affected by thisstripping composition.

(B) A 3×5" steel panel was brush-painted completely with a 4.5 milcoating of Southern Imperial Corp. #1201 polyamide epoxy, the same paintused for panel (2) in part A above. When cured it was found that it took60 sec. to grit blast a 16 sq. cm. area of the panel to white metal withthe blasting equipment described before (80 psi).

The panel was hung in a jar above a mixture of 65% methylene chloride,25% ethylamine, and 10% water by volume. After 30 hours exposure to thevapors of this mixture, the adhesion of the coating to the metal wasalmost completely broken and the coating had swollen considerablywithout tearing much. The panel thus treated appeared to be hanging in aloose plastic bag. When the absorbed vapors were allowed to evaporate,most of the coating shrank back against the panel surface with only afew wrinkles showing; however, very little adhesion was reestablished;one 16 sq. cm. area was blasted to white metal in 12 seconds, another in5 sec., for an average 86% reduction in blasting time.

Example 8A shows the specificity of stripping compositions and Example8A and 8B show that a coating which is substantially unaffected by onevapor phase stripping composition is substantially completelydelaminated by a different vapor phase composition in accordance withthe invention.

EXAMPLE 9

A KTA steel test panel was grit-blasted to white metal and brush-paintedwith two coats of Cook "Phenicon 980" epoxy to an average of 10 mils dryfilm thickness. A photo of a KTA test panel is shown in FIG. 7.1-3 of"Steel Structures Painting Manual", J.D. Keene, Ed., published by theSteel Structures Painting Council, 4400 Fifth Ave., Pittsburgh, Pa.,Vol. 1, 1966, p. 381 and is incorporated herein by reference. It is a 4by 6 inch steel plate to which has been roughly welded a piece of 1/4inch steel 4 in. by 1 in. which had been formed into a square-bottomed"U" shape. An area 7 cm. by 7 cm. which included the protuberance andthe rough welding areas was very difficult to blast, requiring 6.5minutes to blast to 98% white metal using the small gun described beforeat a pressure of 90 psi.

The panel, similarly painted, was exposed to the vapors from a mixtureof methylene chloride (90%) and 85% formic acid (10%) for four days at72° F. and air dried three days. Much of the coating fell off duringdrying. A small vacuum cleaner (G.E. Model MV-1) pulled off all theremaining shattered coating from the 7 cm.×7 cm. area of complexstructure except for one of the inside corners of the "U". The 7 by 7cm. area was 100% cleaned to white metal by 8 seconds of blasting at 90psi. The reduction in blasting time (and grit used) was about 98%.

EXAMPLE 10

A thin layer chromatography chamber containing 200 ml. of a mixture ofmethylene chloride (90%) and 85% formic acid (10%), with blotting papersto speed up the attainment of equilibrium between the liquid and thevapor phases, was cooled to 0° C. in crushed ice in an insulated box. Asand blasted metal panel, coated 5 mils thick with "Phenicon 980", anepoxy paint obtained from Cook Paint and Varnish Company, was suspendedin the vapor space. Within 6 hours, the coating was mainly in the formof large, torn blisters with not more than 20% of the coating stilladhering to the metal. After another 16 hours (overnight - longer thanwas needed), the panel was removed and most of the paint fell off. Therest, except for a few specks, was easily removed by brushing after theabsorbed chemicals had evaporated.

In the same equipment at 72° F. (22° C.), a similar panel was 99%delaminated in 3 hours.

EXAMPLE 11

In a small desiccator containing 100 ml. of a mixture of 70% aqueousethylamine (14%) and methylene chloride (86%), the liquid and vaporphases were separated by a wire screen. Two pine wood panels coveredwith different protective coatings, which had been applied by dipping,were placed on the screen and allowed to stand for two days at 72° F.(22° C.).

Panel (1) was coated with "Flow-Lac Varnish Stain" (Sherwin-Williams);this coating ran off the wood leaving it with only a faint tan stain.

Panel (2) was covered with brown "Rustoleum" (Rust-oleum Corp.) alinseed-menhaden alkyd resin-based paint which had been baked for 24hours at 212° F. (100° C.). This coating blistered and ran off the wood,leaving it clean but slightly stained.

In both cases the wood remained smooth and in good condition, withoutthe "raised grain" effect observed when conventional hot caustic soda isused for stripping.

EXAMPLE 12

The process was scaled up over 1600-fold in an experiment conducted on ahydrostatic paint test tank with interior dimensions 6×6×6 ft (volume1640 gal. or 216 cu. ft.) made by welding six 6×6 steel plates together.It had an exceptionally deep anchor pattern resulting from severalabrasive blastings. The interior of the tank was heavily brush-paintedat the welds and then spray-painted several months before with "Phenicon980", an aromatic amine type epoxy coating made by Cook Paint andVarnish Co. The average dry film thickness on the walls was about 9mils, and the area painted was about 210 sq. ft. (Two 3.4 sq. ft hatcheswere not painted). Besides the hatches, which were sealed with rubbergaskets and bolts, there were two 2 in. openings one in the ceiling inthe front right-hand corner and one in the floor at the rear-rightcorner. These were used to introduce and remove chemical vapors.

Using ordinary one-inch steel pipe, a few pieces of one-inchchemical-resistant hose for flexibility, and a number of elbows and Tsan exterior paint-stripping apparatus was built as follows:

From the top opening steel Ts led to: (1) a mercury U-tube manometer,(2) a vent to the atmosphere connected to 9 ft. of downward sloping pipesurrounded by a 6 in. metal tube which could be filled with coolant and(3) about 10 ft of pipe leading to the suction side of a positivedisplacement gas pump. The pump was a General Motors Model 3-53 from aModel 271 diesel engine modified to accept one inch pipe: It was drivenby a 5 H. P. electric motor equipped for reversing the direction ofshaft rotation and speed control. The pressure side of the pump wasconnected to two steel vessels A and B in series used for evaporatingand later for condensing the chemicals used. The first, made from 8 in.pipe had a useful volume of 5 gal. while the second, made from a 6 in.pipe would hold 2 gal. Each had two one inch pipes leading into the capsand was equipped with a sight glass with neoprene seals. Vessel B wasconnected to the bottom opening of the test tank. The vessels weresitting in metal baths containing 17 gal. of water and equipped withimmersion heaters.

Methylene chloride (4.2 gal, 16 l.) was put into A and 90% formic acid(0.79 gal., 3 l.) into B. The motor was started at about 425 rpm,forcing about 10 cu. ft./min (cfm) of air from the test tank over thesurface of the chemicals in A and B then into the bottom of the testtank, carrying the vapors of the chemicals which evaporated into the airstream. The air-gas stream was recirculated through the system until theconcentration of gases was high enough to wreck the adhesion of thepaint. The water baths were heated to replace the heat of vaporizationof the liquids but the temperature of the gas stream entering the tankwas close to the ambient temperature (34°-70° F., 1°-21° C.) and noevidence of the presence of any liquid stripping composition condensatewas ever observed in the tank during the experiment. Pumping wasinterrupted once to add more methylene chloride.

The gas pump was run for 8 hours at 10 cfm with the vent open. Thecondenser attached to the vent turned out to be underdesigned, and muchof the vapors escaped with the vented air; the amount of each chemicalwhich stayed in the test tank was not determined until later, when thevapors were withdrawn and analyzed. Inspection of the interior of thetank was done at intervals by removing the cap from a 3 in. opening inthe top hatch cover and lowering a light bulb and an adjustable anglemirror with a 2 ft. handle into the tank. At 3 hours the paint was gonefrom a 3 sq. ft. area above the input opening in the floor and damagedin an irregular pattern elsewhere. At 5 hours delamination was extensiveand, from laboratory experience, it seemed probable that enoughchemicals were present to complete the stripping. To make certain thepump was run another 3 hours, when the walls were almost completely bareand pieces of paint were falling like snow from the ceiling. The pumpwas stopped and the tank was loosely closed with valves and corkstoppers. During most of the experiment the open vent kept the pressurein the tank within 1 Torr of atmospheric.

After standing 14 hours a close inspection showed that essentially allthe coating had fallen from the ceiling and walls with the exception ofsome fragments hanging loosely from the metal and some strips in thewelds. This appearance was essentially unchanged during recoveryexperiments (32 hours) and ventilation with fresh air (4 days). When thetank was opened it was found that the loose fragments could be easilyremoved with a compressed air blast, a soft brush, or a vacuum exceptfor a few sq. in. of paint trapped in pockets along the welds, and tinyfragments trapped in some of the pits of the anchor pattern all over thetank. The concensus of opinion of several experts who examined the tankwas that the condition of the surface was much better than a "commercialblast," and for ordinary purposes, could probably be repainted withoutany abrasive blasting, but that for the most demanding jobs, for which anear white metal blast is required, a fast "sweep" with an abrasiveblast was all that would be needed. This was variously estimated torequire 5-15% of the time and material needed for untreated paint.

Recovery of the chemicals absorbed in the paint residue and in the vaporspace of the tank was accomplished without noticeable loss by coolingVessels A and B with ice water and reversing the rotation direction ofthe motor so that gases were sucked out of the bottom opening at 7 cfmand through the cooled vessels. Air entered through the vent and no odorwas detected anywhere. In 6 hrs. 4.7 l. was collected acted after whichthe rate of recovery dropped off rapidly. A total of 6 liters (1.6 gal)was collected in 32 hours. The paint flakes were vacuumed out and foundto weigh 12.5 Kg (27.5 lb) of which about 1.0 Kg (2.2 lb) was formicacid. Analysis of the collected liquids showed that during the strippingthe tank contained a total of 7.1 Kg (15.7 lb) of methylene chloride,1.2 Kg (2.6 lb) of formic acid and 0.15 Kg (0.3 lb) of water. Thus atotal of 8.4 Kg of chemicals stripped 11.5 Kg of paint and 7.4 Kg of thechemicals was recovered.

What we claim as new and desire to be secured by Letters Patent is:
 1. Amethod for stripping a protective coating from the surface of a coatedobject, comprising circulating a stream of a stripping composition in agaseous state at about ambient temperature and pressure into contactwith said surface for a time sufficient to destroy the adhesion betweensaid protective coating and said surface and substantially in theabsence on said surface of liquid condensate of said strippingcomposition, said stripping composition normally being a liquid andhaving a partial pressure of at least about 5 mm. of Hg. at ambienttemperature and pressure.
 2. A method according to claim 1 in which saidstripping composition comprises a lower chloroalkane and the method iscarried out without heating said stripping composition to refluxtemperature.
 3. A method for stripping a protective coating according toclaim 1 and the further step comprising:sealing said surface from theatmosphere in a stripping zone.
 4. A method for stripping a protectivecoating according to claim 3 and the further stepscomprising:withdrawing at least a portion of said air in said strippingzone so that the gas phase in said stripping zone is enriched with saidstripping fluid and circulating said gaseous state stripping compositionwithin said stripping zone.
 5. A method for stripping a protectivecoating according to claim 4 wherein from a major portion tosubstantially all of said air is withdrawn from said stripping zone sothat the atmosphere in the stripping zone is mainly stripping gas, thepressure of which is such that substantially no liquid condensate formson the surface being stripped.
 6. A method for stripping a protectivecoating, according to claim 3 and the further steps of:passing a streamof air over said paint stripping composition in an evaporation zone toproduce a stream of said paint stripping composition in the gaseousphase, introducing the thus formed stream of said stripping compositionin the gaseous phase into said paint stripping zone and withdrawing atleast a portion of the air from said paint stripping zone, whereby thegas phase in said gas stripping zone is enriched with said strippingcomposition.
 7. A method for stripping a protective coating according toclaim 6 wherein from a major portion to substantially all of said air iswithdrawn from said stripping zone so that the atmosphere in thestripping zone is mainly stripping gas, the pressure of which is suchthat substantially no liquid condensate forms on the surface beingstripped.
 8. A method for stripping a protective coating according toclaim 6 and the further steps comprising:continuously withdrawing saidstripping composition in the gaseous state from said stripping zone,continuously recycling said stripping composition in the gas phase whichhas been withdrawn from said stripping zone back to said stripping zoneuntil the adhesion between said protective coating and said surface hasbeen substantially destroyed and then, withdrawing substantially all ofsaid stripping composition in the gaseous state from said strippingzone.
 9. A method according to claim 1 in which said strippingcomposition is comprised of about 25 to 100% by volume of methylenechloride.
 10. A method according to claim 1 in which said paintstripping composition consists essentially of a mixture of methylenechloride as the principal ingredient and at least one additionalcomponent selected from the group consisting of water, ammonia, loweralkylamine, lower carboxylic acid, lower alkanol, lower alkyl ether,lower alkyl ester, lower alkyl nitrile, lower carboxylic acid amide andlower alkyl amide, lower alkane, lower alkyl ketone and benzene andlower-alkyl and halogen substituted benzene, heteroaromatic compoundscontaining up to 8 carbon atoms and volatile inorganic acids, whereinsaid component has a vapor pressure of at least about 5 mm. of Hg atambient temperature.
 11. A method according to claim 10 in which saidstripping composition contains by volume about 70 to 95% of methylenechloride and 5 to 30% of said additional component and wherein saidadditional component includes at least about 1% of water.
 12. A methodaccording to claim 10 in which said stripping composition contains byvolume about 70 to 90% of methylene chloride and 10 to 30% of 33 to 75%aqueous ethylamine.
 13. A method according to claim 10 in which saidstripping composition contains by volume about 70 to 95% of methylenechloride about 1% of water and an additional component selected frommethyl alcohol and methyl ethyl ketone.
 14. In a method of stripping aprotective coating from interior coated surfaces of large constructions,including in particular the interiors of storage tanks on land and holdsand ballast tanks of ships, the steps comprising:sealing said interiorsurface from the atmosphere to form a stripping zone, forming a streamof stripping composition in a gaseous state capable of destroying theadhesion between said coating and said interior surface, said strippingcomposition normally being liquid and having a partial pressure of atleast about 5 mm. of Hg. at ambient temperature and pressure introducingsaid stripping composition in the gaseous state into said stripping zoneinto contact with said interior surface at about ambient temperature andpressure, and at the same time, withdrawing at least a portion of saidair from said stripping zone so that the gas phase in said strippingzone is enriched with said stripping composition, circulating saidstripping composition within said stripping zone and, maintaining saidstripping composition in the gaseous state in contact with said coatedsurface at about ambient temperature and pressure and substantially inthe absence of liquid stripping composition condensate on said surfaceuntil the adhesion between said protective coating and said surface issubstantially destroyed.
 15. A method according to claim 14 in whichsaid stream of stripping composition comprises a lower chloroalkane andis formed without heating said stripping composition to refluxtemperature and wherein form a major portion to substantially all ofsaid air is withdrawn from said stripping zone so that the atmosphere inthe stripping zone is mainly stripping gas, the pressure of which issuch that substantially no liquid condensate forms on the surface beingstripped.
 16. A method according to claim 14 and the further steps ofcontinuously withdrawing a portion of said gaseous state strippingcomposition from said stripping zone and recirculating said withdrawngaseous state stripping composition back into said stripping zone.
 17. Amethod according to claim 14 in which said stream of strippingcomposition in the gaseous state is formed by passing a stream of airover said stripping composition in an evaporation zone and withoutheating said stripping composition to reflux temperature.
 18. A methodaccording to claim 1 in which said stripping composition comprises achlorinated hydrocarbon.
 19. A method according to claim 1 in which saidprotective coating is paint.
 20. A method according to claim 1 in whichsaid protective coating comprises epoxy paint.
 21. A method according toclaim 1 in which said protective coating is residual crude oil.
 22. Amethod according to claim 1 in which said protective coating is Bunker Cfuel oil.